1. Field of the Invention
This invention relates to a booster for elevating the pressure level of a pressurized fluid to be supplied to a load through pipings or the like.
2. Description of the Prior Art
Various pneumatically operating units in a plant or factory usually receive a supply of compressed air from a common pressurized air source through pipings. In a case where one or some of the units require supply of compressed air of a higher pressure level than the source pressure or in a case where the pressure of compressed air unavoidably drops before reaching the end of the pipings, it is necessary to install further pressurized air sources or intensify the air pressure in the pipings by means of a booster or the like.
The present inventors previously proposed a booster suitable for use in such a case, which is adapted to elevate the air pressure by the use of line air pressure in the pipings alone without necessitating power supply from outside. As a result of further studies and pilot operations, this previously proposed booster turned out to have still a number of problems as follows.
As diagrammatically shown in FIG. 1, the booster is provided with a pair of coaxial cylinders 5a and 5b on opposite sides of a center partition wall 4 of a casing 1 with inlet and outlet ports 2 and 3. Pistons 6a and 6b which are fitted in the cylinders 5a and 5b are connected with each other by a rod 7 which is hermetically passed through the partition wall 4. The boosting chambers 9a and 9b which are respectively defined in the cylinders 5a and 5b on the opposite sides of the center partition wall 4 by the pistons 6a and 6b are communicated with the inlet port 2 through inlet check valves 11a and 11b which permit only air flows into the boosting chambers and with the outlet port 3 through outlet check valves 12a and 12b which permit only air flows out of the boosting chambers. A switch valve 13 which communicates drive chambers 10a and 10b in the cylinders 5a and 5b alternately with the inlet port 2 and an exhaust ports 17a or 17b is provided in the partition wall 4, push rods 18a and 18b of the switch valve being protruded into the boosting chambers 9a and 9b, respectively, to switch the position of the switch valve 13 alternately by pushing actions of the pistons 6a and 6b.
In FIG. 1, indicated at 15 is a pressure regulator valve which regulates the pressure to be supplied to the drive chambers 10a and 10b.
With the booster of the above-described construction, the air pressure from the inlet port 2 is constantly drawn into the boosting chambers 9a and 9b through the inlet check valves 11a and 11b, and, when the switch valve 13 is pushed rightward by the pushing action of the piston 6b of the left cylinder as shown in FIG. 1, the pressurized air from the pressure regulator valve 15 is fed to the right drive chamber 10a through the switch valve 13 while releasing the left drive chamber 10b to the atmosphere. Accordingly, the pistons 6a and 6b are driven leftward by the fluid pressure in the drive chamber 10a, boosting the air pressure in the boosting chamber 9a for supply through the outlet port 3. As soon as the piston 6a comes to its stroke end, pushing the push rod 18a to change the position of the switch valve 13, the above-described operation is reversed to intensify the pressure in the boosting chamber 9b.
The booster of this sort can shift the position of the switch valve 13 by the reciprocating movements of the pistons without troubles as long as the pistons are in high-speed operation. However, when the secondary pressure reaches a predetermined level due to reduced consumption thereof or when the driving speed of the pistons is lowered due to a drop in supply pressure, there may arise a difficult situation that the pistons which have once come to a stop would not re-start due to malfunctioning of the switch valve.
Our study in this respect revealed that, if the pushing force of the pistons is lowered due to a drop in the supply pressure or by other reason when the switch valve is shifted near to its neutral position, the switch valve which is now free of the switching action of the pistons stalls at the neutral position. Consequently, even if the supply air pressure is increased again, the switch valve remains in the neutral position without restarting since the pressurized air is supplied to neither one of the drive chambers through the switch valve.
More particularly, the failure of re-start takes place, for example, when the piston 6a of FIG. 1 under influence of a dropped fluid pressure pushes the spool of the switch valve 13 to an intermediate position to switch the charging and discharging of the drive chambers 10a and 10b. At this time, the piston 6a is slided in an opposite direction (to the right in FIG. 1) by a reversed small operating force. However, in a case where a differential type check valve having no springs is used for the inlet check valve, the charging and discharging of the drive chambers 10a and 10b are not yet sufficient at a time point soon after switching of the spool, so that a pressure differential sufficient for closing the inlet check valve 11b is not yet produced in the boosting chamber 9b by rightward slide of the pistons 6a and 6b. As a consequence the inlet check valve 11b remains open, and no pressure is produced in the boosting chamber 9b.
Accordingly, the pistons 6a and 6b are slided by inertia force even when there is almost no working fluid pressure in the drive chamber 10b, slightly sliding the spool of the switch valve 13 through the push rod 18b. As a result, the switch valve 13 stops at the neutral position in some cases, failing to re-start as the fluid pressure is not supplied to the drive chambers 10a and 10b even after the line air pressure is restored.